Neck options of the Arba Kanfot Tzitzit, Abra Kanfos Tzitzis for kids and adults
The round neck is also known as oval neck, scoop neck *1, U neck, square neck, and more.
The round neck has an advantage that it isn't too visible through the shirt.
The slit neck is also known as flap neck, V neck (because when the flaps are open, it has a v-shape), T neck (because when the flaps are closed, it looks like a T), cut neck, and more. It is also sometimes referred to as a portrait neckline.
The slit neck has the advantage that you can fold in the flaps, and get a wider neck.
The slit neck is more popular.
According to some, the slit neck is more Mehudar.
*1 ^ (A scoop neck shirt is one in which the scoop-shape neckline is dropped significantly below normal limits.)